Process for separation of CO2 from CO2 -containing gases

ABSTRACT

For separating CO 2  from CO 2  -containing gases, especially stack gases and/or blast furnace gases, dimethylformamide is employed as a physical scrubbing medium to ensure high CO 2  purity. After absorption of CO 2 , the DMF is regenerated and returned into the scrubbing stage. Dimethylformamide is utilized as the scrubbing medium.

BACKGROUND OF THE INVENTION

This invention relates to a process for the separation of CO₂ from CO₂-containing gases, especially stack or flue gases and/or blast furnacegases, by scrubbing with a physical scrubbing medium which is thereafterregenerated to remove CO₂, and is then recycled into the scrubbingstage.

Several methods are known for obtaining CO₂ from gaseous mixtures.Chemical and physical scrubbing operations, as well as adsorptiveseparating methods have distinguished themselves as the importantprocesses. Important chemical scrubbing operations include, for example,the NH₃ or monoethanolamine (MEA) scrubbing processes, whereas themethanol scrubbing process conducted at low temperatures is important asa physical scrubbing operation. Adsorptive removal of CO₂ from gaseousmixtures can be effected with the aid of activated carbon or a molecularsieve. Another possibility for separating CO₂ from gaseous mixtures isthe regenerator method employing the periodic application of a vacuum topurge the regenerator of CO₂.

Though the technology of removing CO₂ from industrial gases is indeedvery old, all the conventional processes exhibit one or moredisadvantages. For example, the chemical scrubbing methods require forregeneration a very high thermal input, in the form of low-pressuresteam (≧2.5 bar), for example, and such methods initially yield awater-saturated gas which, due to danger of corrosion, must be driedbefore being further compressed (generally to 140 bar). Physicalscrubbing with methanol, as the absorbent, due to the high vaporpressure of methanol may result in very high losses of scrubbing medium.In this process, the stack gas, at ambient pressure, would either haveto be compressed to high pressures, or the process would have to beconducted under extremely low temperatures to maintain losses ofmethanol within economical limits. However, both measures entailexcessive costs in energy. Finally, the regenerator process yields a gashaving too high a proportion of N₂ which would have to be subsequentlyseparated at high expense in order to obtain sufficiently pure CO₂.

SUMMARY

It is therefore an object of the present invention to provide animproved process of the type discussed hereinabove, especially a processwherein deficiencies of the prior art are diminished or eliminated. Afurther object is to provide a CO₂ product of high purity.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

These objects are attained according to the invention by utilizingdimethylformamide as the scrubbing medium.

Dimethylformamide (DMF) has proven to be suitable for the separation ofCO₂ from gases containing at least about 8, preferably at least about 8to 30 CO₂ in percent by volume, especially from stack gases and/or blastfurnace gases. The vapor pressure of DMF is lower by two powers of tenthan that of methanol, but it has practically the same high solubilityfor CO₂ as methanol. In this connection, the use of DMF as the scrubbingmedium offers the substantial advantage of lowering the losses and costsfor the operating medium while simultaneously maintaining a level ofinitial investment costs which, at least, are no higher than when usingmethanol. Moreover, since the CO₂ is obtained as an anhydrous gas, thereis no need for drying the thus-separated CO₂ as is the case, forexample, in chemical scrubbing operations. Another advantage in usingDMF resides in that gases having any level of sulfur content (SO₂) canbe processed without the occurrence of any problems in the scrubbingprocess or any contamination of the CO₂ product. The reason for this isthat DMF possesses an extremely high solubility for SO₂ withsimultaneously a very high selectivity. Any SO₂ contained in the gas canbe transferred out by regeneration of a minimum partial stream of DMF.This partial stream, containing about 1% SO₂ by weight will handled in asmall additional column, heated with steam under low pressure. Thesoluted SO₂ will be taken out on the top of this column as a SO₂ -richgasstream. This SO₂ stream can be used for chemicals (H₂ SO₄, elementalsulfur) or can be liquified for transportation.

The scrubbing process with the use of DMF is carried out according to apreferred embodiment under a pressure of 1.5-3 bar and at a temperatureof -30° to -60° C. Thus, the stack gas which, in most cases, is notpresent under pressure, need only be compressed to a moderately highpressure.

According to a preferred embodiment of the invention, regeneration ofthe scrubbing medium is conducted by pressure reducing and heating thescrubbing medium. In this process, the regeneration is advantageouslyconducted under vacuum, specifically under a pressure of 0.05-0.3 bar,and at a temperature of between -60° and -10° C. By operating under suchconditions, it is possible to obtain a product purity of CO₂ of up to99% by volume.

The CO₂ liberated during regeneration is then compressed in a pluralityof compression steps, e.g., 2, preferably 4 steps, suitably to, intotal, about 80 to 200, especially about 140 bar. According to anotherpreferred embodiment of the process of this invention, the loadedscrubbing medium is heated in heat exchange with a partial stream of theproduct CO₂, the latter being at a medium pressure, e.g., about 6 to 20,especially about 6 to 15 bar, so that the medium-pressure CO₂ isliquefied during this step. After expansion by pressure reduction, theliquefied CO₂ is vaporized and discharged. In a preferred aspect of theinvention, the CO₂ can be vaporized in a cooling coil arranged in thescrubbing column, whereby the heat of solution released duringabsorption of CO₂ in the scrubbing medium is removed. The removal of theCO₂ product by the use of a vacuum, and the medium-pressure auxiliaryCO₂ cycle is preferably integrated into the necessary CO₂ compressionsystem.

According to another preferred embodiment of this invention, the CO₂-containing gas is cooled by means of reversible regenerators prior tothe scrubbing step. The regenerators are filled with acid-resistantheat-storage regenerator packing material of a conventional kind.

The pure CO₂ obtained by the process of this invention can be utilizedespecially for injection into petroleum reservoirs. In general, theprocess of this invention provides CO₂ gas of at least 99.5, andpreferably at least 98% by volume.

BRIEF DESCRIPTION OF DRAWING

The attached drawing is a FIGURE depicting a preferred embodiment of theinvention in schematic form.

DETAILED DESCRIPTION

Stack gas under ambient pressure containing about 13.5 vol-% of CO₂ isintroduced via conduit 1 and compressed to 2.4 bar in a compressor 2.The stack gas is available at a temperature of 130°-150° C. In aregenerator system 3, the stack gas is cooled conventionally to -50° C.The regenerator system 3 consists, in the present example, of twomutually separated, reversible and interchangeably connectedregenerators. While, for example, in regenerator (A) stack gas is cooledoff on the regenerator packing in a flow direction from the top towardthe bottom, the stack gas freed of CO₂ is reheated via a conduit 4 whileflowing through regenerator (B) in a direction from the bottom (thetemperature level of the scrubbing process) toward the top to atemperature of about 120° C. and is discharged via a conduit 5. The flowthrough the regenerators is cyclically reversed after a switching timeof, for example, three minutes.

The stack gas, cooled to -50° C., is fed via a conduit 6 to the lowersection of a scrubbing column 7 and freed of CO₂ by physical absorption.The scrubbing medium employed is dimethylformamide in accordance withthis invention. This scrubbing medium is introduced via conduit 8 intothe upper section of the scrubbing column 7. The stack gas is thus freedof CO₂ countercurrently to downwardly flowing DMF and can be withdrawnfrom the head of the scrubbing column 7, expanded in turbine 9, heatedin heat exchanger 10, with regenerated scrubbing medium to be cooled,and fed via conduit 4 to the regenerator system 3. The ratio of solventto CO₂ is about 30 to 50 tons solvent to one ton of CO₂. The number oftheoretical plates in the scrubber is between 8 to 20.

The loaded scrubbing medium is withdrawn from the sump of scrubbingcolumn 7 via conduit 11 and expanded in valve 12 to about atmosphericpressure. During this step, any concomitantly dissolved N₂ is removed inthe gaseous phase; this N₂ is withdrawn from a separator 13 via conduit14 and fed, together with the regenerated stack gas, in conduit 4 to theregenerator system 3.

The remaining scrubbing medium is withdrawn via conduit 15 fromseparator 13 and expanded via valve 16. During expansion, CO₂ isliberated, present under a pressure of 0.1 bar and removed by means of afan 17 in conduit 18 from a separator 19. The partially regeneratedscrubbing medium is conducted to a further separator 20 and heatedtherein in heat exchange with a partial stream of the product CO₂ fromconduit 21 to -20° C. During this step, additional CO₂ is driven off inthe gaseous phase and is admixed via conduit 22 to the CO₂ product fromthe first separator. The heated and completely regenerated DMF iswithdrawn via conduit 22a and cooled in separator 19 in heat exchangewith DMF to be regenerated, and again introduced into the scrubbingcolumn 7 by way of conduit 8.

The product CO₂ in conduit 18 is compressed in compressor 23 to about5.7 bar, in compressor 24 to 20 bar and in compressor 25 to 140 bar anddischarged.

The partial stream of product CO₂ branched off via conduit 21 is thusunder a medium pressure of 20 bar at a temperature of about 30° C. Thismedium-pressure CO₂ is liquefied in heat exchange with regenerated DMF,withdrawn via conduit 26 and, after expansion, via valve 27, to 6 bar,vaporized in a cooling coil 28 arranged in the scrubbing column 7. Inthis way, the heat of solution liberated during CO₂ absorption in theDMF is removed. Finally, the resultant preheated partial stream of CO₂is conducted through the regenerator system 3 for further heating, andfed, at a temperature of 20° C. and under a pressure of 5.7 bar viaconduit 29 to the CO₂ product upstream of the compressor 24.

If the stack gas contains also SO₂ besides CO₂, this SO₂ is likewisedissolved in the DMF. Therefore, the provision is made to withdraw apartial stream of the partially regenerated DMF via conduit 30 andtransfer SO₂ out of this stream.

A comparison of the costs for operating media when using a chemicalscrubbing operation--an NH₃ scrubbing process has been selected as theexample--and when using the DMF scrubbing operation of this invention isset forth below. In both cases, a raw gas containing 13.5 vol-% of CO₂is the starting material. The product obtained is 10,000 Nm³ /h of pureCO₂ (99 vol-%) under a pressure of 140 bar.

    ______________________________________                                                     NH.sub.3 Scrubbing                                                                       DMF Scrubbing                                         ______________________________________                                        Electrical Energy                                                             MW @ DM 80     4.0 MW =     7.7 MW =                                                         DM 320 per hour                                                                            DM 616 per hour                                   Cooling Water                                                                 m.sup.3 @ DM 0.05                                                                            2,415 m.sup.3 h =                                                                          435 m.sup.3 /h =                                                 DM 121 per hour                                                                            DM 22 per hour                                    Low-Pressure Steam                                                            2.5 bar                                                                       tons/h @ DM 12 40 tons/h =  --                                                               DM 480 per hour                                                                            --                                                NH.sub.3 or DMF                                                               kg/h NH.sub.3 DM 0.30 per kg                                                                 35 kg/h =                                                                     DM 10 per hour                                                 DMF DM 3.00 per kg          15 kg/h =                                                                     DM 45 per hour                                    Costs for Operating                                                           Medium                                                                        DM/h           931          683                                               Pfennigs per Nm.sup.3 of CO.sub.2                                                            9.3          6.8                                               ______________________________________                                    

As can be derived from the comparison, considerable savings in costs canbe achieved by employing the process of the present invention.

Typical blast furnace and stack gases treatable by this invention havethe following approximate ranges of analyses:

    ______________________________________                                        Vol %        blast furnace                                                                            stackgas                                              ______________________________________                                        N.sub.2      56         80                                                    CO           20         --                                                    O.sub.2      --          6                                                    CO.sub.2     22         14                                                    ______________________________________                                    

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

I claim:
 1. In a process for separating CO₂ from a CO₂ -containing gas,said process comprising scrubbing said gas in a scrubbing stage withdimethylformamide to absorb CO₂, regenerating resultant loadeddimethylformamide in a regeneration stage to remove CO₂, and compressingthe CO₂ to medium and then to high pressure in a plurality of pressurestages, wherein the regeneration stage is conducted by pressurereduction and heating of the loaded dimethylformamide and recyclingregenerated dimethylformamide into the scrubbing stage, the improvementwhich comprises heating the loaded dimethylformamide in heat exchangewith a partial stream of the product CO₂ at a medium pressure, therebyliquefying the medium-pressure CO₂.
 2. A process according to claim 1,wherein said scrubbing stage is conducted under a pressure of 1.5-3 barand at a temperature of -30° to -60° C.
 3. A process according to claim2, wherein the regeneration of the scrubbing medium is conducted under apressure of 0.05-0.3 bar and at a temperature of between -60° C. and-10° C.
 4. A process according to claim 2, comprising recompressing theCO₂ liberated during regeneration in a plurality of compression stages,including a medium pressure level and a high pressure level.
 5. Aprocess according to claim 1, wherein the liquefied CO₂ is expanded andthen vaporized in a cooling coil in heat exchange with thedimethylformamide in the scrubbing stage, so as to remove the heat ofabsorption of the CO₂ within the scrubbing stage.
 6. A process accordingto claim 1, wherein the CO₂ -containing gas also contains SO₂, and apartial stream of partially regenerated dimethylformamide is furtherregenerated in a separate stage to remove SO₂ by heating at a highertemperature than the CO₂ -removal regeneration stage.
 7. A processaccording to claim 1, further comprising pressure reducing, vaporizing,and withdrawing the liquefied CO₂.
 8. A process according to claim 1,further comprising, prior to the scrubbing step, cooling the CO₂-containing gas reversible regenerators.
 9. A process according to claim1, wherein the CO₂ -containing gas to be scrubbed contains at leastabout 8 percent by volume.
 10. A process according to claim 9, whereinthe CO₂ -containing gas is a stack gas.
 11. A process according to claim9, wherein the CO₂ -containing gas is a blast furnace gas.
 12. In aprocess for separating CO₂ from a CO₂ -containing gas, said processcomprising scrubbing said gas in a scrubbing stage withdimethylformamide to absorb CO₂, regenerating resultant loadeddimethylformamide in a regeneration stage to remove CO₂, wherein theregeneration stage is conducted by pressure reduction and heating of theloaded dimethylformamide and recycling regenerated dimethylformamideinto the scrubbing stage, the improvement which comprises heating theloaded dimethylformamide in heat exchange with a partial stream of theproduct CO₂ thereby liquefying the CO₂, and expanding the liquified CO₂and then vaporizing it in a cooling coil in heat exchange with thedimethylformamide in the scrubbing stage, so as to remove the heat ofabsorption of the CO₂ within the scrubbing stage.
 13. In a process forseparating CO₂ from a CO₂ and SO₂ -containing gas, said processcomprising scrubbing said gas in a scrubbing stage withdimethylformamide to absorb CO₂, regenerating resultant loadeddimethylformamide in a regeneration stage to remove CO₂, wherein theregeneration stage is conducted by pressure reduction and heating of theloaded dimethylformamide and recycling regenerated dimethylformamideinto the scrubbing stage, the improvement which comprises heating apartial stream of partially regenerated dimethylformamide to remove SO₂in a separate stage at a higher temperature than the CO₂ -regenerationstage and heating the remaining loaded dimethylformamide in heatexchange with a partial stream of the product CO₂ thereby liquefying theCO₂.